“The branch of physical chemistry, which deals the nature
of surfaces and also with the chemical and physical
processes which takes place on the surfaces, is called
In surface chemistry, we study the phenomenon of
adsorption, catalysis and colloidal properties.
(1) Definition: The phenomenon of attracting and retaining
the molecules of a substance on the surface of a liquid
or solid resulting in to higher concentration of the
molecules on the surface is called adsorption.
(2) Causes of adsorption: Unbalanced forces of attraction
or free valencies which is present at the solid or liquid
surface, have the property to attract and retain the
molecules of a gas or a dissolved substance on to their
surfaces with which they come in contact.
Example: Ammonia gas placed in contact with
charcoal gets adsorbed on the charcoal whereas
ammonia gas placed in contact with water gets
absorbed into water,
(3) Difference between adsorption and absorption:
It is a surface
It concerns with the
whole mass of the
In it, the substance is
It implies that a
only retained on the
substance is uniformly
surface and does not
distributed, through the
go into the bulk or
body of the solid or
interior of the solid or
In it the concentration
In it the concentration
of the adsorbed
molecules is always
greater at the free
It is rapid in the
It occurs at the uniform
beginning and slows
down near the
Examples : (i) Water
Examples : (i) Water
vapours adsorbed by
vapours absorbed by
(ii) NH3 is adsorbed by
(ii) NH3 is absorbed in
water forming NH4OH
(4) Surface forces: Only the surface atoms of an adsorbent
play an active role in adsorption. These atoms posses
unbalanced forces of various types such as, Vander
Waal‟s forces and chemical bond forces.
Thus, the residual force-field on a free surface
which is responsible for adsorption is produced. For
example, when a solid substance is broken into two
pieces, two new surfaces are formed and therefore, the
number of unbalanced forces becomes more. As a
result the tendency for adsorption becomes large.
(5) Reversible and Irreversible adsorption: The adsorption
is reversible, if the adsorbate can be easily removed
from the surface of the adsorbent by physical methods.
If the adsorbate can not be easily removed from the
surface of the adsorbent is called irreversible
(6) Characteristics of adsorption:
Adsorption refers to the existence of a higher
concentration of any particular component at the
surface of a liquid or a solid phase.
Adsorption is accompanied by decrease in the ΔG
(free energy change) of the system when ΔG = 0,
adsorption equilibrium is said to be established.
(iii) Adsorption is invariably accompanied by evolution
of heat, i.e. it is an exothermic process. In other
words, ΔH of adsorption is always negative.
(iv) When a gas is adsorbed, the freedom of
movement of its molecules becomes restricted. On
account of it decrease in the entropy of the gas
after adsorption, i.e. ΔS is negative.
For a process to be spontaneous, the
thermodynamic requirement is that ΔG must be
negative, i.e. there is decrease in free energy. On
the basis of Gibb‟s Helmholtz equation, ΔG = ΔH –
T ΔS, ΔG can be negative if ΔH has sufficiently
high negative value and T ΔS has positive value.
Classification of adsorption
Adsorption can be classified into two categories as
(1) Depending upon the concentration: In adsorption the
concentration of one substance is different at the
surface of the other substance as compared to
adjoining bulk or interior phase.
Positive adsorption: If the concentration of
adsorbate is more on the surface as compared to
its concentration in the bulk phase then it is called
Example: When a concentrated solution of KCl
is shaken with blood charcoal, it shows positive
Negative adsorption: If the concentration of the
adsorbate is less than its concentration in the bulk
then it is called negative adsorption.
Example : When a dilute solution of KCl is
shaken with blood charcoal, it shows negative
(2) Depending upon the nature of force existing between
adsorbate molecule and adsorbent
Physical adsorption : If the forces of attraction
existing between adsorbate and adsorbent are
Vander Waal‟s forces, the adsorption is called
physical adsorption. This type of adsorption is
also known as Physisorption or Vander Waal‟s
adsorption. It can be easily reversed by heating or
decreasing the pressure.
Chemical adsorption : If the forces of attraction
existing between adsorbate particles and
adsorbent are almost of the same strength as
chemical bonds, the adsorption is called chemical
adsorption. This type of adsorption is also called
as Chemisorption or Langmuir adsorption. This
type of adsorption cannot be easily reversed.
Factors which affect the extent of adsorption
The following are the factors which affect the adsorption,
(1) Nature of the adsorbate (gas) and adsorbent (solid)
In general, easily liquefiable gases e.g., CO2, NH3,
Cl2 and SO2 etc. are adsorbed to a greater extent
than the elemental gases e.g. H2, O2, N2, He etc.
(while Chemisorption is specific in nature.)
Porous and finely powdered solid e.g. charcoal,
fullers earth, adsorb more as compared to the
hard non-porous materials. Due to this property
powdered charcoal is used in gas masks.
(2) Surface area of the solid adsorbent
The extent of adsorption depends directly upon
the surface area of the adsorbent, i.e. larger the
surface area of the adsorbent, greater is the
extent of adsorption.
Surface area of a powdered solid adsorbent
depends upon its particle size. Smaller the particle
size, greater is its surface area.
(3) Effect of pressure on the adsorbate gas
An increase in the pressure of the adsorbate gas
increases the extent of adsorption.
At low temperature, the extent of adsorption
increases rapidly with pressure.
(iii) Small range of pressure, the extent of adsorption
is found to be directly proportional to the
(iv) At high pressure (closer to the saturation vapour
pressure of the gas), the adsorption tends to
achieve a limiting value.
(4) Effect of temperature
As adsorption is accompanied by evolution of
heat, so according to the Le-Chatelier‟s principle,
the magnitude of adsorption should decrease with
rise in temperature.
The relationship between the extent of adsorption
and temperature at any constant pressure is called
(iii) A physical adsorption isobar shows a decrease in
x/m (where „m‟ is the mass of the adsorbent and
„x‟ that of adsorbate) as the temperature rises.
(iv) The isobar of Chemisorption show an increase in
the beginning and then decrease as the
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